This is supposedly because a struct with a member qualified with
a const effectively deletes its default copy constructor, which
is considered as being user-defined somehow. Not sure where
exactly is the phrase in the C++98 standard who allows such C /
C++ incompatibility though.
workaround for C++ 98 const union problem.
Not the case of recent compilers, but compilers before C++11
rejected ruby.h, like https://ci.appveyor.com/project/ruby/ruby/builds/27225706/job/qjca7dpe204dytbd
This is supposedly because a struct with a member qualified with
a const effectively deletes its default copy constructor, which
is considered as being user-defined somehow. Not sure where
exactly is the phrase in the C++98 standard who allows such C /
C++ incompatibility though.